Network Working Group A. Keranen
Internet-Draft Ericsson
Intended status: Informational M. Kovatsch
Expires: April 26, 2019 Siemens AG
K. Hartke
Ericsson
October 23, 2018
RESTful Design for Internet of Things Systems
draft-irtf-t2trg-rest-iot-02
Abstract
This document gives guidance for designing Internet of Things (IoT)
systems that follow the principles of the Representational State
Transfer (REST) architectural style. This document is a product of
the IRTF Thing-to-Thing Research Group (T2TRG).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 26, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Architecture . . . . . . . . . . . . . . . . . . . . . . 6
3.2. System design . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Uniform Resource Identifiers (URIs) . . . . . . . . . . . 9
3.4. Representations . . . . . . . . . . . . . . . . . . . . . 10
3.5. HTTP/CoAP Methods . . . . . . . . . . . . . . . . . . . . 11
3.5.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5.2. POST . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5.3. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5.4. DELETE . . . . . . . . . . . . . . . . . . . . . . . 13
3.5.5. FETCH . . . . . . . . . . . . . . . . . . . . . . . . 13
3.5.6. PATCH . . . . . . . . . . . . . . . . . . . . . . . . 13
3.6. HTTP/CoAP Status/Response Codes . . . . . . . . . . . . . 13
4. REST Constraints . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Client-Server . . . . . . . . . . . . . . . . . . . . . . 14
4.2. Stateless . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3. Cache . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.4. Uniform Interface . . . . . . . . . . . . . . . . . . . . 16
4.5. Layered System . . . . . . . . . . . . . . . . . . . . . 17
4.6. Code-on-Demand . . . . . . . . . . . . . . . . . . . . . 17
5. Hypermedia-driven Applications . . . . . . . . . . . . . . . 18
5.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. Knowledge . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3. Interaction . . . . . . . . . . . . . . . . . . . . . . . 19
5.4. Hypermedia-driven Design Guidance . . . . . . . . . . . . 20
6. Design Patterns . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Collections . . . . . . . . . . . . . . . . . . . . . . . 20
6.2. Calling a Procedure . . . . . . . . . . . . . . . . . . . 21
6.2.1. Instantly Returning Procedures . . . . . . . . . . . 21
6.2.2. Long-running Procedures . . . . . . . . . . . . . . . 21
6.2.3. Conversion . . . . . . . . . . . . . . . . . . . . . 22
6.2.4. Events as State . . . . . . . . . . . . . . . . . . . 22
6.3. Server Push . . . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Future Work . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
The Representational State Transfer (REST) architectural style [REST]
is a set of guidelines and best practices for building distributed
hypermedia systems. At its core is a set of constraints, which when
fulfilled enable desirable properties for distributed software
systems such as scalability and modifiability. When REST principles
are applied to the design of a system, the result is often called
RESTful and in particular an API following these principles is called
a RESTful API.
Different protocols can be used with RESTful systems, but at the time
of writing the most common protocols are HTTP [RFC7230] and CoAP
[RFC7252]. Since RESTful APIs are often simple and lightweight, they
are a good fit for various IoT applications. The goal of this
document is to give basic guidance for designing RESTful systems and
APIs for IoT applications and give pointers for more information.
Design of a good RESTful IoT system has naturally many commonalities
with other Web systems. Compared to other systems, the key
characteristics of many IoT systems include:
o need to accommodate for constrained devices, so with IoT, REST is
not only used for scaling out (large number of clients on a web
server), but also for scaling down (efficient server on
constrained node)
o data formats, interaction patterns, and other mechanisms that
minimize, or preferably avoid, the need for human interaction
o preference for compact and simple data formats to facilitate
efficient transfer over (often) constrained networks and
lightweight processing in constrained nodes
o the usually large number of endpoints can not be updated
simultaneously, yet the system needs to be able to evolve in the
field without long downtimes
2. Terminology
This section explains some of the common terminology that is used in
the context of RESTful design for IoT systems. For terminology of
constrained nodes and networks, see [RFC7228].
Cache: A local store of response messages and the subsystem that
controls storage, retrieval, and deletion of messages in it.
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Client: A node that sends requests to servers and receives
responses. In RESTful IoT systems it's common for nodes to have
more than one role (e.g., both server and client; see
Section 3.1).
Client State: The state kept by a client between requests. This
typically includes the currently processed representation, the set
of active requests, the history of requests, bookmarks (URIs
stored for later retrieval), and application-specific state (e.g.,
local variables). (Note that this is called "Application State"
in [REST], which has some ambiguity in modern (IoT) systems where
the overall state of the distributed application (i.e.,
application state) is reflected in the union of all Client States
and Resource States of all clients and servers involved.)
Content Negotiation: The practice of determining the "best"
representation for a client when examining the current state of a
resource. The most common forms of content negotiation are
Proactive Content Negotiation and Reactive Content Negotiation.
Dereference: To use an access mechanism (e.g., HTTP or CoAP) to
perform an action on a URI's resource.
Dereferencable URI: A URI that can be dereferenced, i.e., an action
can be performed on the identified resource. Not all HTTP or CoAP
URIs are dereferencable, e.g., when the target resource does not
exist.
Form: A hypermedia control that enables a client to change the state
of a resource or to construct a query locally.
Forward Proxy: An intermediary that is selected by a client, usually
via local configuration rules, and that can be tasked to make
requests on behalf of the client. This may be useful, for
example, when the client lacks the capability to make the request
itself or to service the response from a cache in order to reduce
response time, network bandwidth, and energy consumption.
Gateway: A reverse proxy that provides an interface to a non-RESTful
system such as legacy systems or alternative technologies such as
Bluetooth ATT/GATT. See also "Reverse Proxy".
Hypermedia Control: A component, such as a link or a form, embedded
in a representation that identifies a resource for future
hypermedia interactions. If the client engages in an interaction
with the identified resource, the result may be a change to
resource state and/or client state.
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Idempotent Method: A method where multiple identical requests with
that method lead to the same visible resource state as a single
such request.
Link: A hypermedia control that enables a client to navigate between
resources and thereby change the client state.
Link Relation Type: An identifier that describes how the link target
resource relates to the current resource (see [RFC5988]).
Media Type: A string such as "text/html" or "application/json" that
is used to label representations so that it is known how the
representation should be interpreted and how it is encoded.
Method: An operation associated with a resource. Common methods
include GET, PUT, POST, and DELETE (see Section 3.5 for details).
Origin Server: A server that is the definitive source for
representations of its resources and the ultimate recipient of any
request that intends to modify its resources. In contrast,
intermediaries (such as proxies caching a representation) can
assume the role of a server, but are not the source for
representations as these are acquired from the origin server.
Proactive Content Negotiation: A content negotiation mechanism where
the server selects a representation based on the expressed
preference of the client. For example, an IoT application could
send a request to a sensor with preferred media type "application/
senml+json".
Reactive Content Negotiation: A content negotiation mechanism where
the client selects a representation from a list of available
representations. The list may, for example, be included by a
server in an initial response. If the user agent is not satisfied
by the initial response representation, it can request one or more
of the alternative representations, selected based on metadata
(e.g., available media types) included in the response.
Representation: A serialization that represents the current or
intended state of a resource and that can be transferred between
clients and servers. REST requires representations to be self-
describing, meaning that there must be metadata that allows peers
to understand which representation format is used. Depending on
the protocol needs and capabilities, there can be additional
metadata that is transmitted along with the representation.
Representation Format: A set of rules for serializing resource
state. On the Web, the most prevalent representation format is
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HTML. Other common formats include plain text and formats based
on JSON [RFC7159], XML, or RDF. Within IoT systems, often compact
formats based on JSON, CBOR [RFC7049], and EXI
[W3C.REC-exi-20110310] are used.
Representational State Transfer (REST): An architectural style for
Internet-scale distributed hypermedia systems.
Resource: An item of interest identified by a URI. Anything that
can be named can be a resource. A resource often encapsulates a
piece of state in a system. Typical resources in an IoT system
can be, e.g., a sensor, the current value of a sensor, the
location of a device, or the current state of an actuator.
Resource State: A model of a resource's possible states that is
represented in a supported representation type, typically a media
type. Resources can change state because of REST interactions
with them, or they can change state for reasons outside of the
REST model.
Resource Type: An identifier that annotates the application-
semantics of a resource (see Section 3.1 of [RFC6690]).
Reverse Proxy: An intermediary that appears as a server towards the
client but satisfies the requests by forwarding them to the actual
server (possibly via one or more other intermediaries). A reverse
proxy is often used to encapsulate legacy services, to improve
server performance through caching, and to enable load balancing
across multiple machines.
Safe Method: A method that does not result in any state change on
the origin server when applied to a resource.
Server: A node that listens for requests, performs the requested
operation and sends responses back to the clients.
Uniform Resource Identifier (URI): A global identifier for
resources. See Section 3.3 for more details.
3. Basics
3.1. Architecture
The components of a RESTful system are assigned one or both of two
roles: client or server. Note that the terms "client" and "server"
refer only to the roles that the nodes assume for a particular
message exchange. The same node might act as a client in some
communications and a server in others. Classic user agents (e.g.,
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Web browsers) are always in the client role and have the initiative
to issue requests. Origin servers always have the server role and
govern over the resources they host. Simple IoT devices, such as
sensors and actuators, are commonly acting as servers and exposing
their physical world interaction capabilities (e.g., temperature
measurement or door lock control capability) as resources. Typical
IoT system client can be a cloud service that retrieves data from the
sensors and commands the actuators based on the sensor information.
Alternatively an IoT data storage system could work as a server where
IoT sensor devices send data, in client role.
________ _________
| | | |
| User (C)-------------------(S) Origin |
| Agent | | Server |
|________| |_________|
(Browser) (Web Server)
Figure 1: Client-Server Communication
Intermediaries (such as forward proxies, reverse proxies, and
gateways) implement both roles, but only forward requests to other
intermediaries or origin servers. They can also translate requests
to different protocols, for instance, as CoAP-HTTP cross-proxies.
________ __________ _________
| | | | | |
| User (C)---(S) Inter- (C)--------------------(S) Origin |
| Agent | | mediary | | Server |
|________| |__________| |_________|
(Browser) (Forward Proxy) (Web Server)
Figure 2: Communication with Forward Proxy
Reverse proxies are usually imposed by the origin server. In
addition to the features of a forward proxy, they can also provide an
interface for non-RESTful services such as legacy systems or
alternative technologies such as Bluetooth ATT/GATT. In this case,
reverse proxies are usually called gateways. This property is
enabled by the Layered System constraint of REST, which says that a
client cannot see beyond the server it is connected to (i.e., it is
left unaware of the protocol/paradigm change).
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________ __________ _________
| | | | | |
| User (C)--------------------(S) Inter- (x)---(x) Origin |
| Agent | | mediary | | Server |
|________| |__________| |_________|
(Browser) (Gateway) (Legacy System)
Figure 3: Communication with Reverse Proxy
Nodes in IoT systems often implement both roles. Unlike
intermediaries, however, they can take the initiative as a client
(e.g., to register with a directory, such as CoRE Resource Directory
[I-D.ietf-core-resource-directory], or to interact with another
thing) and act as origin server at the same time (e.g., to serve
sensor values or provide an actuator interface).
________ _________
| | | |
| Thing (C)-------------------------------------(S) Origin |
| (S) | Server |
|________| \ |_________|
(Sensor) \ ________ (Resource Directory)
\ | |
(C) Thing |
|________|
(Controller)
Figure 4: Constrained RESTful environments
3.2. System design
When designing a RESTful system, the primary effort goes into
modeling the state of the distributed application and assigning it to
the different components (i.e., clients and servers). How clients
can navigate through the resources and modify state to achieve their
goals is defined through hypermedia controls, that is, links and
forms. Hypermedia controls span a kind of a state machine where the
nodes are resources and the transitions are links or forms. Clients
run this state machine (i.e., the application) by retrieving
representations, processing the data, and following the included
hypermedia controls. In REST, remote state is changed by submitting
forms. This is usually done by retrieving the current state,
modifying the state on the client side, and transferring the new
state to the server in the form of new representations - rather than
calling a service and modifying the state on the server side.
Client state encompasses the current state of the described state
machine and the possible next transitions derived from the hypermedia
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controls within the currently processed representation (see
Section 2). Furthermore, clients can have part of the state of the
distributed application in local variables.
Resource state includes the more persistent data of an application
(i.e., independent of individual clients). This can be static data
such as device descriptions, persistent data such as system
configurations, but also dynamic data such as the current value of a
sensor on a thing.
It is important to distinguish between "client state" and "resource
state" and keep them separate. Following the Stateless constraint,
the client state must be kept only on clients. That is, there is no
establishment of shared information about past and future
interactions between client and server (usually called a session).
On the one hand, this makes requests a bit more verbose since every
request must contain all the information necessary to process it. On
the other hand, this makes servers efficient and scalable, since they
do not have to keep any state about their clients. Requests can
easily be distributed over multiple worker threads or server
instances. For IoT systems, this constraint lowers the memory
requirements for server implementations, which is particularly
important for constrained servers (e.g., sensor nodes) and servers
serving large amount of clients (e.g., Resource Directory).
3.3. Uniform Resource Identifiers (URIs)
An important part of RESTful API design is to model the system as a
set of resources whose state can be retrieved and/or modified and
where resources can be potentially also created and/or deleted.
Uniform Resource Identifiers (URIs) are used to indicate a resource
for interaction, to reference a resource from another resource, to
advertise or bookmark a resource, or to index a resource by search
engines.
foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
A URI is a sequence of characters that matches the syntax defined in
[RFC3986]. It consists of a hierarchical sequence of five
components: scheme, authority, path, query, and fragment (from most
significant to least significant). A scheme creates a namespace for
resources and defines how the following components identify a
resource within that namespace. The authority identifies an entity
that governs part of the namespace, such as the server
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"www.example.org" in the "http" scheme. A host name (e.g., a fully
qualified domain name) or an IP address, potentially followed by a
transport layer port number, are usually used in the authority
component for the "http" and "coap" schemes. The path and query
contain data to identify a resource within the scope of the URI's
scheme and naming authority. The fragment allows to refer to some
portion of the resource, such as a Record in a SenML Pack. However,
fragments are processed only at client side and not sent on the wire.
[RFC7320] provides more details on URI design and ownership with best
current practices for establishing URI structures, conventions, and
formats.
For RESTful IoT applications, typical schemes include "https",
"coaps", "http", and "coap". These refer to HTTP and CoAP, with and
without Transport Layer Security (TLS) [RFC5246]. (CoAP uses
Datagram TLS (DTLS) [RFC6347], the variant of TLS for UDP.) These
four schemes also provide means for locating the resource; using the
HTTP protocol for "http" and "https", and with the CoAP protocol for
"coap" and "coaps". If the scheme is different for two URIs (e.g.,
"coap" vs. "coaps"), it is important to note that even if the rest of
the URI is identical, these are two different resources, in two
distinct namespaces.
Some schemes are for URIs with main purpose as identifiers and hence
are not dereferencable, e.g., the "urn" scheme can be used to
construct unique names in registered namespaces. In particular the
"urn:dev" [I-D.ietf-core-dev-urn] details multiple ways for
generating and representing endpoint identifiers of IoT devices.
The query parameters can be used to parametrize the resource. For
example, a GET request may use query parameters to request the server
to send only certain kind data of the resource (i.e., filtering the
response). Query parameters in PUT and POST requests do not have
such established semantics and are not commonly used. Whether the
order of the query parameters matters in URIs is unspecified and they
can be re-ordered e.g., by proxies. Therefore applications should
not rely on their order; see Section 3.3 of [RFC6943] for more
details.
3.4. Representations
Clients can retrieve the resource state from an origin server or
manipulate resource state on the origin server by transferring
resource representations. Resource representations have a media type
that tells how the representation should be interpreted by
identifying the representation format used.
Typical media types for IoT systems include:
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o "text/plain" for simple UTF-8 text
o "application/octet-stream" for arbitrary binary data
o "application/json" for the JSON format [RFC7159]
o "application/cbor" for CBOR [RFC7049]
o "application/exi" for EXI [W3C.REC-exi-20110310]
o "application/senml+json" and "application/senml+cbor" for Sensor
Measurement Lists (SenML) data [RFC8428]
A full list of registered Internet Media Types is available at the
IANA registry [IANA-media-types] and numerical media types registered
for use with CoAP are listed at CoAP Content-Formats IANA registry
[IANA-CoAP-media].
3.5. HTTP/CoAP Methods
Section 4.3 of [RFC7231] defines the set of methods in HTTP;
Section 5.8 of [RFC7252] defines the set of methods in CoAP. As part
of the Uniform Interface constraint, each method can have certain
properties that give guarantees to clients.
Safe methods do not cause any state change on the origin server when
applied to a resource. For example, the GET method only returns a
representation of the resource state but does not change the
resource. Thus, it is always safe for a client to retrieve a
representation without affecting server-side state.
Idempotent methods can be applied multiple times to the same resource
while causing the same visible resource state as a single such
request. For example, the PUT method replaces the state of a
resource with a new state; replacing the state multiple times with
the same new state still results in the same state for the resource.
However, the response from the server can be different when the same
idempotent method is used multiple times. For example when DELETE is
used twice on an existing resource, the first request would remove
the association and return success acknowledgement whereas the second
request would likely result in error response due to non-existing
resource.
The following lists the most relevant methods and gives a short
explanation of their semantics.
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3.5.1. GET
The GET method requests a current representation for the target
resource, while the origin server must ensure that there are no side-
effects on the resource state. Only the origin server needs to know
how each of its resource identifiers corresponds to an implementation
and how each implementation manages to select and send a current
representation of the target resource in a response to GET.
A payload within a GET request message has no defined semantics.
The GET method is safe and idempotent.
3.5.2. POST
The POST method requests that the target resource process the
representation enclosed in the request according to the resource's
own specific semantics.
If one or more resources has been created on the origin server as a
result of successfully processing a POST request, the origin server
sends a 201 (Created) response containing a Location header field
(with HTTP) or Location-Path and/or Location-Query Options (with
CoAP) that provide an identifier for the resource created. The
server also includes a representation that describes the status of
the request while referring to the new resource(s).
The POST method is not safe nor idempotent.
3.5.3. PUT
The PUT method requests that the state of the target resource be
created or replaced with the state defined by the representation
enclosed in the request message payload. A successful PUT of a given
representation would suggest that a subsequent GET on that same
target resource will result in an equivalent representation being
sent.
The fundamental difference between the POST and PUT methods is
highlighted by the different intent for the enclosed representation.
The target resource in a POST request is intended to handle the
enclosed representation according to the resource's own semantics,
whereas the enclosed representation in a PUT request is defined as
replacing the state of the target resource. Hence, the intent of PUT
is idempotent and visible to intermediaries, even though the exact
effect is only known by the origin server.
The PUT method is not safe, but is idempotent.
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3.5.4. DELETE
The DELETE method requests that the origin server remove the
association between the target resource and its current
functionality.
If the target resource has one or more current representations, they
might or might not be destroyed by the origin server, and the
associated storage might or might not be reclaimed, depending
entirely on the nature of the resource and its implementation by the
origin server.
The DELETE method is not safe, but is idempotent.
3.5.5. FETCH
The CoAP-specific FETCH method [RFC8132] requests a representation of
a resource parameterized by a representation enclosed in the request.
The fundamental difference between the GET and FETCH methods is that
the request parameters are included as the payload of a FETCH
request, while in a GET request they're typically part of the query
string of the request URI.
The FETCH method is safe and idempotent.
3.5.6. PATCH
The PATCH method [RFC5789] [RFC8132] requests that a set of changes
described in the request entity be applied to the target resource.
The PATCH method is not safe nor idempotent.
The CoAP-specific iPATCH method is a variant of the PATCH method that
is not safe, but is idempotent.
3.6. HTTP/CoAP Status/Response Codes
Section 6 of [RFC7231] defines a set of Status Codes in HTTP that are
used by application to indicate whether a request was understood and
satisfied, and how to interpret the answer. Similarly, Section 5.9
of [RFC7252] defines the set of Response Codes in CoAP.
The status codes consist of three digits (e.g., "404" with HTTP or
"4.04" with CoAP) where the first digit expresses the class of the
code. Implementations do not need to understand all status codes,
but the class of the code must be understood. Codes starting with 1
are informational; the request was received and being processed.
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Codes starting with 2 indicate a successful request. Codes starting
with 3 indicate redirection; further action is needed to complete the
request. Codes stating with 4 and 5 indicate errors. The codes
starting with 4 mean client error (e.g., bad syntax in the request)
whereas codes starting with 5 mean server error; there was no
apparent problem with the request, but server was not able to fulfill
the request.
Responses may be stored in a cache to satisfy future, equivalent
requests. HTTP and CoAP use two different patterns to decide what
responses are cacheable. In HTTP, the cacheability of a response
depends on the request method (e.g., responses returned in reply to a
GET request are cacheable). In CoAP, the cacheability of a response
depends on the response code (e.g., responses with code 2.04 are
cacheable). This difference also leads to slightly different
semantics for the codes starting with 2; for example, CoAP does not
have a 2.00 response code whereas 200 ("OK") is commonly used with
HTTP.
4. REST Constraints
The REST architectural style defines a set of constraints for the
system design. When all constraints are applied correctly, REST
enables architectural properties of key interest [REST]:
o Performance
o Scalability
o Reliability
o Simplicity
o Modifiability
o Visibility
o Portability
The following sub-sections briefly summarize the REST constraints and
explain how they enable the listed properties.
4.1. Client-Server
As explained in the Architecture section, RESTful system components
have clear roles in every interaction. Clients have the initiative
to issue requests, intermediaries can only forward requests, and
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servers respond requests, while origin servers are the ultimate
recipient of requests that intent to modify resource state.
This improves simplicity and visibility (also for digital forensics),
as it is clear which component started an interaction. Furthermore,
it improves modifiability through a clear separation of concerns.
In IoT systems, endpoints often assume both roles of client and
(origin) server simultaneously. When an IoT device has initiative
(because there is a user, e.g., pressing a button, or installed
rules/policies), it acts as a client. When a device offers a
service, it is in server role.
4.2. Stateless
The Stateless constraint requires messages to be self-contained.
They must contain all the information to process it, independent from
previous messages. This allows to strictly separate the client state
from the resource state.
This improves scalability and reliability, since servers or worker
threads can be replicated. It also improves visibility because
message traces contain all the information to understand the logged
interactions. Furthermore, the Stateless constraint enables caching.
For IoT, the scaling properties of REST become particularly
important. Note that being self-contained does not necessarily mean
that all information has to be inlined. Constrained IoT devices may
choose to externalize metadata and hypermedia controls using Web
linking, so that only the dynamic content needs to be sent and the
static content such as schemas or controls can be cached.
4.3. Cache
This constraint requires responses to have implicit or explicit
cache-control metadata. This enables clients and intermediary to
store responses and re-use them to locally answer future requests.
The cache-control metadata is necessary to decide whether the
information in the cached response is still fresh or stale and needs
to be discarded.
Cache improves performance, as less data needs to be transferred and
response times can be reduced significantly. Less transfers also
improves scalability, as origin servers can be protected from too
many requests. Local caches furthermore improve reliability, since
requests can be answered even if the origin server is temporarily not
available.
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Caching usually only makes sense when the data is used by multiple
participants. In the IoT, however, it might make sense to cache also
individual data to protect constrained devices. Security often
hinders the ability to cache responses. For IoT systems, object
security may be preferable over transport layer security, as it
enables intermediaries to cache responses while preserving security.
4.4. Uniform Interface
All RESTful APIs use the same, uniform interface independent of the
application. This simple interaction model is enabled by exchanging
representations and modifying state locally, which simplifies the
interface between clients and servers to a small set of methods to
retrieve, update, and delete state - which applies to all
applications.
In contrast, in a service-oriented RPC approach, all required ways to
modify state need to be modeled explicitly in the interface resulting
in a large set of methods - which differs from application to
application. Moreover, it is also likely that different parties come
up with different ways how to modify state, including the naming of
the procedures, while the state within an application is a bit easier
to agree on.
A REST interface is fully defined by:
o URIs to identify resources
o representation formats to represent and manipulate resource state
o self-descriptive messages with a standard set of methods (e.g.,
GET, POST, PUT, DELETE with their guaranteed properties)
o hypermedia controls within representations
The concept of hypermedia controls is also known as HATEOAS:
Hypermedia As The Engine Of Application State. The origin server
embeds controls for the interface into its representations and
thereby informs the client about possible next requests. The mostly
used control for RESTful systems is Web Linking [RFC5590].
Hypermedia forms are more powerful controls that describe how to
construct more complex requests, including representations to modify
resource state.
While this is the most complex constraints (in particular the
hypermedia controls), it improves many different key properties. It
improves simplicity, as uniform interfaces are easier to understand.
The self-descriptive messages improve visibility. The limitation to
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a known set of representation formats fosters portability. Most of
all, however, this constraint is the key to modifiability, as
hypermedia-driven, uniform interfaces allow clients and servers to
evolve independently, and hence enable a system to evolve.
For a large number of IoT applications, the hypermedia controls are
mainly used for the discovery of resources, as they often serve
sensor data. Such resources are "dead ends", as they usually do not
link any further and only have one form of interaction: fetching the
sensor value. For IoT, the critical parts of the Uniform Interface
constraint are the descriptions of messages and representation
formats used. Simply using, for instance, "application/json" does
not help machine clients to understand the semantics of the
representation. Yet defining very precise media types limits the re-
usability and interoperability. Representation formats such as SenML
[RFC8428] try to find a good trade-off between precision and re-
usability. Another approach is to combine a generic format such as
JSON with syntactic as well as semantic annotations (see
[I-D.handrews-json-schema-validation] and [W3C-TD], resp.).
4.5. Layered System
This constraint enforces that a client cannot see beyond the server
with which it is interacting.
A layered system is easier to modify, as topology changes become
transparent. Furthermore, this helps scalability, as intermediaries
such as load balancers can be introduced without changing the client
side. The clean separation of concerns helps with simplicity.
IoT systems greatly benefit from this constraint, as it allows to
effectively shield constrained devices behind intermediaries and is
also the basis for gateways, which are used to integrate other (IoT)
ecosystems.
4.6. Code-on-Demand
This principle enables origin servers to ship code to clients.
Code-on-Demand improves modifiability, since new features can be
deployed during runtime (e.g., support for a new representation
format). It also improves performance, as the server can provide
code for local pre-processing before transferring the data.
As of today, code-on-demand has not been explored much in IoT
systems. Aspects to consider are that either one or both nodes are
constrained and might not have the resources to host or dynamically
fetch and execute such code. Moreover, the origin server often has
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no understanding of the actual application a mashup client realizes.
Still, code-on-demand can be useful for small polyfills, e.g., to
decode payloads, and potentially other features in the future.
5. Hypermedia-driven Applications
Hypermedia-driven applications take advantage of hypermedia controls,
i.e., links and forms, embedded in the resource representations. A
hypermedia client is a client that is capable of processing these
hypermedia controls. Hypermedia links can be used to give additional
information about a resource representation (e.g., the source URI of
the representation) or pointing to other resources. The forms can be
used to describe the structure of the data that can be sent (e.g.,
with a POST or PUT method) to a server, or how a data retrieval
(e.g., GET) request for a resource should be formed. In a
hypermedia-driven application the client interacts with the server
using only the hypermedia controls, instead of selecting methods and/
or constructing URIs based on out-of-band information, such as API
documentation.
5.1. Motivation
The advantage of this approach is increased evolvability and
extensibility. This is important in scenarios where servers exhibit
a range of feature variations, where it's expensive to keep evolving
client knowledge and server knowledge in sync all the time, or where
there are many different client and server implementations.
Hypermedia controls serve as indicators in capability negotiation.
In particular, they describe available resources and possible
operations on these resources using links and forms, respectively.
There are multiple reasons why a server might introduce new links or
forms:
o The server implements a newer version of the application. Older
clients ignore the new links and forms, while newer clients are
able to take advantage of the new features by following the new
links and submitting the new forms.
o The server offers links and forms depending on the current state.
The server can tell the client which operations are currently
valid and thus help the client navigate the application state
machine. The client does not have to have knowledge which
operations are allowed in the current state or make a request just
to find out that the operation is not valid.
o The server offers links and forms depending on the client's access
control rights. If the client is unauthorized to perform a
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certain operation, then the server can simply omit the links and
forms for that operation.
5.2. Knowledge
A client needs to have knowledge of a couple of things for successful
interaction with a server. This includes what resources are
available, what representations of resource states are available,
what each representation describes, how to retrieve a representation,
what state changing operations on a resource are possible, how to
perform these operations, and so on.
Some part of this knowledge, such as how to retrieve the
representation of a resource state, is typically hard-coded in the
client software. For other parts, a choice can often be made between
hard-coding the knowledge or acquiring it on-demand. The key to
success in either case is the use in-band information for identifying
the knowledge that is required. This enables the client to verify
that is has all required knowledge and to acquire missing knowledge
on-demand.
A hypermedia-driven application typically uses the following
identifiers:
o URI schemes that identify communication protocols,
o Internet Media Types that identify representation formats,
o link relation types or resource types that identify link
semantics,
o form relation types that identify form semantics,
o variable names that identify the semantics of variables in
templated links, and
o form field names that identify the semantics of form fields in
forms.
The knowledge about these identifiers as well as matching
implementations have to be shared a priori in a RESTful system.
5.3. Interaction
A client begins interacting with an application through a GET request
on an entry point URI. The entry point URI is the only URI a client
is expected to know before interacting with an application. From
there, the client is expected to make all requests by following links
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and submitting forms that are provided in previous responses. The
entry point URI can be obtained, for example, by manual configuration
or some discovery process (e.g., DNS-SD [RFC6763] or Resource
Directory [I-D.ietf-core-resource-directory]). For Constrained
RESTful environments "/.well-known/core" relative URI is defined as a
default entry point for requesting the links hosted by servers with
known or discovered addresses [RFC6690].
5.4. Hypermedia-driven Design Guidance
Assuming self-describing representation formats (i.e., human-readable
with carefully chosen terms or processible by a formatting tool) and
a client supporting the URI scheme used, a good rule of thumb for a
good hypermedia-driven design is the following: A developer should
only need an entry point URI to drive the application. All further
information how to navigate through the application (links) and how
to construct more complex requests (forms) are published by the
server(s). There must be no need for additional, out-of-band
information (e.g., API specification).
For machines, a well-chosen set of information needs to be shared a
priori to agree on machine-understandable semantics. Agreeing on the
exact semantics of terms for relation types and data elements will of
course also help the developer. [I-D.hartke-core-apps] proposes a
convention for specifying the set of information in a structured way.
6. Design Patterns
Certain kinds of design problems are often recurring in variety of
domains, and often re-usable design patterns can be applied to them.
Also some interactions with a RESTful IoT system are straightforward
to design; a classic example of reading a temperature from a
thermometer device is almost always implemented as a GET request to a
resource that represents the current value of the thermometer.
However, certain interactions, for example data conversions or event
handling, do not have as straightforward and well established ways to
represent the logic with resources and REST methods.
The following sections describe how common design problems such as
different interactions can be modeled with REST and what are the
benefits of different approaches.
6.1. Collections
A common pattern in RESTful systems across different domains is the
collection. A collection can be used to combine multiple resources
together by providing resources that consist of set of (often
partial) representations of resources, called items, and links to
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resources. The collection resource also defines hypermedia controls
for managing and searching the items in the collection.
Examples of the collection pattern in RESTful IoT systems are the
CoRE Resource Directory [I-D.ietf-core-resource-directory], CoAP pub/
sub broker [I-D.ietf-core-coap-pubsub], and resource discovery via
".well-known/core". Collection+JSON [CollectionJSON] is an example
of a generic collection Media Type.
6.2. Calling a Procedure
To modify resource state, clients usually use GET to retrieve a
representation from the server, modify that locally, and transfer the
resulting state back to the server with a PUT (see Section 4.4).
Sometimes, however, the state can only be modified on the server
side, for instance, because representations would be too large to
transfer or part of the required information shall not be accessible
to clients. In this case, resource state is modified by calling a
procedure (or "function"). This is usually modeled with a POST
request, as this method leaves the behavior semantics completely to
the server. Procedure calls can be divided into two different
classes based on how long they are expected to execute: "instantly"
returning and long-running.
6.2.1. Instantly Returning Procedures
When the procedure can return within the expected response time of
the system, the result can be directly returned in the response. The
result can either be actual content or just a confirmation that the
call was successful. In either case, the response does not contain a
representation of the resource, but a so-called action result.
Action results can still have hypermedia controls to provide the
possible transitions in the application state machine.
6.2.2. Long-running Procedures
When the procedure takes longer than the expected response time of
the system, or even longer than the response timeout, it is a good
pattern to create a new resource to track the "task" execution. The
server would respond instantly with a "Created" status (HTTP code 201
or CoAP 2.01) and indicate the location of the task resource in the
corresponding header field (or CoAP option) or as a link in the
action result. The created resource can be used to monitor the
progress, to potentially modify queued tasks or cancel tasks, and to
eventually retrieve the result.
Monitoring information would be modeled as state of the task
resource, and hence be retrievable as representation. The result -
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when available - can be embedded in the representation or given as a
link to another sub-resource. Modifying tasks can be modeled with
forms that either update sub-resources via PUT or do a partial write
using PATCH or POST. Canceling a task would be modeled with a form
that uses DELETE to remove the task resource.
6.2.3. Conversion
A conversion service is a good example where REST resources need to
behave more like a procedure call. The knowledge of converting from
one representation to another is located only at the server to
relieve clients from high processing or storing lots of data. There
are different approaches that all depend on the particular conversion
problem.
As mentioned in the previous sections, POST request are a good way to
model functionality that does not necessarily affect resource state.
When the input data for the conversion is small and the conversion
result is deterministic, however, it can be better to use a GET
request with the input data in the URI query part. The query is
parameterizing the conversion resource, so that it acts like a look-
up table. The benefit is that results can be cached also for HTTP
(where responses to POST are not cacheable). In CoAP, cacheability
depends on the response code, so that also a response to a POST
request can be made cacheable through a 2.05 Content code.
When the input data is large or has a binary encoding, it is better
to use POST requests with a proper Media Type for the input
representation. A POST request is also more suitable, when the
result is time-dependent and the latest result is expected (e.g.,
exchange rates).
6.2.4. Events as State
In event-centric paradigms such as pub/sub, events are usually
represented by an incoming message that might even be identical for
each occurrence. Since the messages are queued, the receiver is
aware of each occurrence of the event and can react accordingly. For
instance, in an event-centric system, ringing a door bell would
result in a message being sent that represents the event that it was
rung.
In resource-oriented paradigms such as REST, messages usually carry
the current state of the remote resource, independent from the
changes (i.e., events) that have lead to that state. In a naive yet
natural design, a door bell could be modeled as a resource that can
have the states unpressed and pressed. There are, however, a few
issues with this approach. Polling is not an option, as it is highly
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unlikely to be able to observe the pressed state with any realistic
polling interval. When using CoAP Observe with Confirmable
notifications, the server will usually send two notifications for the
event that the door bell was pressed: notification for changing from
unpressed to pressed and another one for changing back to unpressed.
If the time between the state changes is very short, the server might
drop the first notification, as Observe only guarantees only eventual
consistency (see Section 1.3 of [RFC7641]).
The solution is to pick a state model that fits better to the
application. In the case of the door bell - and many other event-
driven resources - the solution could be a counter that counts how
often the bell was pressed. The corresponding action is taken each
time the client observes a change in the received representation.
In the case of a network outage, this could lead to a ringing sound
10 minutes after the bell was rung. Also including a timestamp of
the last counter increment in the state can help to suppress ringing
a sound when the event has become obsolete.
6.3. Server Push
Overall, a universal mechanism for server push, that is, change-of-
state notifications and stand-alone event notifications, is still an
open issue that is being discussed in the Thing-to-Thing Research
Group. It is connected to the state-event duality problem and
custody transfer, that is, the transfer of the responsibility that a
message (e.g., event) is delivered successfully.
A proficient mechanism for change-of-state notifications is currently
only available for CoAP: Observing resources [RFC7641]. It offers
enventual consistency, which guarantees "that if the resource does
not undergo a new change in state, eventually all registered
observers will have a current representation of the latest resource
state". It intrinsically deals with the challenges of lossy
networks, where notifications might be lost, and constrained
networks, where there might not be enough bandwidth to propagate all
changes.
For stand-alone event notifications, that is, where every single
notification contains an identifiable event that must not be lost,
observing resources is not a good fit. A better strategy is to model
each event as a new resource, whose existence is notified through
change-of-state notifications of an index resource (cf. Collection
pattern). Large numbers of events will cause the notification to
grow large, as it needs to contain a large number of Web links.
Blockwise transfers [RFC7959] can help here. When the links are
ordered by freshness of the events, the first block can already
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contain all links to new events. Then, observers do not need to
retrieve the remaining blocks from the server, but only the
representations of the new event resources.
An alternative pattern is to exploit the dual roles of IoT devices,
in particular when using CoAP: they are usually client and server at
the same time. A client observer would subscribe to events by
registering a callback URI at the origin server, e.g., using a POST
request and receiving the location of a temporary subscription
resource as handle. The origin server would then publish events by
sending POST requests containing the event to the observer. The
cancellation can be modeled through deleting the subscription
resource. This pattern makes the origin server responsible for
delivering the event notifications. This goes beyond retransmissions
of messages; the origin server is usually supposed to queue all
undelivered events and to retry until successful delivery or explicit
cancellation. In HTTP, this pattern is known as REST Hooks.
In HTTP, there exist a number of workarounds to enable server push,
e.g., long polling and streaming [RFC6202] or server-sent events
[W3C.REC-html5-20141028]. Long polling as an extension that both
server and client need to be aware of. In IoT systems, long polling
can introduce a considerable overhead, as the request has to be
repeated for each notification. Streaming and server-sent events (in
fact an evolved version of streaming) are more efficient, as only one
request is sent. However, there is only one response header and
subsequent notifications can only have content. There are no means
for individual status and metadata, and hence no means for proficient
error handling (e.g., when the resource is deleted).
7. Security Considerations
This document does not define new functionality and therefore does
not introduce new security concerns. We assume that system designers
apply classic Web security on top of the basic RESTful guidance given
in this document. Thus, security protocols and considerations from
related specifications apply to RESTful IoT design. These include:
o Transport Layer Security (TLS): [RFC5246] and [RFC6347]
o Internet X.509 Public Key Infrastructure: [RFC5280]
o HTTP security: Section 9 of [RFC7230], Section 9 of [RFC7231],
etc.
o CoAP security: Section 11 of [RFC7252]
o URI security: Section 7 of [RFC3986]
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IoT-specific security is mainly work in progress at the time of
writing. First specifications include:
o (D)TLS Profiles for the Internet of Things: [RFC7925]
Further IoT security considerations are available in
[I-D.irtf-t2trg-iot-seccons].
8. Acknowledgement
The authors would like to thank Mert Ocak, Heidi-Maria Back, Tero
Kauppinen, Michael Koster, Robby Simpson, Ravi Subramaniam, Dave
Thaler, Erik Wilde, and Niklas Widell for the reviews and feedback.
9. References
9.1. Normative References
[I-D.ietf-core-dev-urn]
Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
Names for Device Identifiers", draft-ietf-core-dev-urn-03
(work in progress), October 2018.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-15 (work in
progress), August 2018.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-17 (work in progress), October 2018.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D. Dissertation,
University of California, Irvine , 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
.
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[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
.
[RFC5590] Harrington, D. and J. Schoenwaelder, "Transport Subsystem
for the Simple Network Management Protocol (SNMP)",
STD 78, RFC 5590, DOI 10.17487/RFC5590, June 2009,
.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
.
[RFC6202] Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
"Known Issues and Best Practices for the Use of Long
Polling and Streaming in Bidirectional HTTP", RFC 6202,
DOI 10.17487/RFC6202, April 2011,
.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, .
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, .
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
.
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[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
.
[W3C.REC-exi-20110310]
Schneider, J. and T. Kamiya, "Efficient XML Interchange
(EXI) Format 1.0", World Wide Web Consortium
Recommendation REC-exi-20110310, March 2011,
.
[W3C.REC-html5-20141028]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O'Connor, T., and S. Pfeiffer, "HTML5",
World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014,
.
9.2. Informative References
[CollectionJSON]
Amundsen, M., "Collection+JSON - Document Format",
February 2013,
.
[I-D.handrews-json-schema-validation]
Wright, A., Andrews, H., and G. Luff, "JSON Schema
Validation: A Vocabulary for Structural Validation of
JSON", draft-handrews-json-schema-validation-01 (work in
progress), March 2018.
[I-D.hartke-core-apps]
Hartke, K., "CoRE Applications", draft-hartke-core-apps-08
(work in progress), October 2018.
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-05 (work in
progress), July 2018.
[I-D.irtf-t2trg-iot-seccons]
Garcia-Morchon, O., Kumar, S., and M. Sethi, "State-of-
the-Art and Challenges for the Internet of Things
Security", draft-irtf-t2trg-iot-seccons-15 (work in
progress), May 2018.
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[IANA-CoAP-media]
"CoAP Content-Formats", n.d.,
.
[IANA-media-types]
"Media Types", n.d., .
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, .
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, .
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
.
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[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
.
[RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
Bormann, "Sensor Measurement Lists (SenML)", RFC 8428,
DOI 10.17487/RFC8428, August 2018,
.
[W3C-TD] Kaebisch, S. and T. Kamiya, "Web of Things (WoT) Thing
Description", April 2018,
.
Appendix A. Future Work
o Interface semantics: shared knowledge among system components (URI
schemes, media types, relation types, well-known locations; see
core-apps)
o Unreliable (best effort) communication, robust communication in
network with high packet loss, 3-way commit
o Discuss directories, such as CoAP Resource Directory
o More information on how to design resources; choosing what is
modeled as a resource, etc.
Authors' Addresses
Ari Keranen
Ericsson
Jorvas 02420
Finland
Email: ari.keranen@ericsson.com
Matthias Kovatsch
Siemens AG
Otto-Hahn-Ring 6
Munich D-81739
Germany
Email: matthias.kovatsch@siemens.com
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Klaus Hartke
Ericsson
Torshamnsgatan 23
Stockholm SE-16483
Sweden
Email: klaus.hartke@ericsson.com
Keranen, et al. Expires April 26, 2019 [Page 30]